Monodentate configuration

In the case of the bridged complexes, the process involves changing from a bidentate to a monodentate configuration. For these systems the mode of transformation is variable. In close-packed crystals the rearrangement is a first-order process, ie, it occurs discontinuously at a fixed pressure. For slightly less close-packed crystals the transformation occurs over some range of pressure, eg, 2—3 GPa (20—30 kbar). In the language of physics the process... 

It has 6-coordination with a chelating acetate [106] and may be converted (reversibly) into Ru(OAc)2(PPh3)3, which has the/ac-configuration with one monodentate and one bidentate acetate. It is fluxional at room temperature but at —70°C the phosphines are non-equivalent on the NMR timescale [107],... 

Several forms are imaginable for the [Ni°(butadiene)2L] and [Ni°(butadiene)J active catalysts, depending on the monodentate (p2) or the bidentate (p4) coordination mode of butadiene from either its s-cis or its s-trans configuration. The two butadienes can be coordinated in bis(p2), p4, p2, and bis(p4) modes for the PR3/P(OR)3-stabilized catalyst complex, giving rise to formal 16e, 18e, and 20e species. On the other hand, bis(p4)- and p4,p2-butadiene species and also tris(p2)- and p4,bis(p2)-butadiene compounds are possible species for the [Ni°(butadiene)2] and [Ni°(butadiene)3] forms for the [Ni°(butadiene)J active catalyst. In general, for butadiene to coordinate in a bidentate fashion, the p4-cis mode is thermodynamically favorable relative to the p4-trans mode, while the p2-trans mode prevails for monodentate coordination. 

As bidentates seemed to give more stable complexes, and because trans configurations seemed to be favoured by some of the monodentates, Thomas and Suss-Fink investigated the use of ligands that are bidentate and can coordinate in a trans fashion [11,16], This has led to the development of new diphosphines based on condensation reactions of 2-diphenylphos-phinobenzoic acid with aminoalcohols or diols, reminiscent of the wide bite angle ligands used by Trost for the asymmetric allylic alkylation (Chapter 13.2). The best... 

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